The present invention relates to a double-feed sheet detection apparatus for detecting a double-feed of sheets of paper in a printer and the like which feeds sheets one by one sequentially.
Conventionally, a double-feed sheet detection apparatus, which uses a light-transmission sensor, is known. FIG. 5 is a block diagram of a conventional double-feed sheet detection apparatus. A paper transport path is constructed between a light emitting lamp 1 and a light receiving sensor 2. Output from the light receiving sensor 2 is amplified by an amplifier 3 and converted from analog to digital by an analog-to-digital (A/D) converter 4. The converted output is then input through an input/output (I/0) circuit 51 to a central processing unit(CPU) 61. The gain of the amplifier 3 is controlled by a gain control circuit 91, and the intensity of the light from the light emitting lamp 1 is controlled by a lamp control circuit 100.
FIGS. 6A and 6B illustrate a flow chart showing operational steps performed in a conventional double-feed sheet detection apparatus. In the case of using the conventional apparatus, an operator judges the thickness of a sheet of paper and sets a switch at an optimum signal level for the thickness of the sheet. Thereafter, the apparatus stores a level of the first sheet. The apparatus then judges whether the number of sheets being fed is one or more than one, based on the stored level.
The operation process will be explained according to the above-mentioned flow chart. The process includes the following steps:
in Step 200, an operator sets a sheet of paper in a paper feeder;
in Step 201, the operator judges whether or not the sheet is thick;
in Step 202, if the operator judges the sheet is not thick in the former step, he judges whether or not the sheet is thin;
in Step 203, if the operator judges the sheet is not thin, he sets a switch at an optimum level for a standard sheet of paper;
in Step 215, when the operator judges the sheet is thick in Step 201, he sets the switch at an optimum level for a thick sheet and goes on to Step 204;
in Step 217, when the operator judges the sheet is thin in Step 202, he sets the switch at an optimum level for a thin sheet and goes on to Step 204;
in Step 204, feeding of the sheets is started;
in Step 205, it is judged whether or not a sheet is fed;
in Step 206, if it is judged that the sheet is fed in the former step, a level of the fed paper is measured;
in Step 207, a level of double-feed is set based on the measurement result;
in Step 208, it is judged whether or not the sheet is ejected;
in Step 209, if it is judged that the sheet is ejected in the former step, it is judged whether or not a subsequent sheet is fed;
in Step 210, if it is judged that the subsequent sheet is fed in the former step, the level of the sheet is measured;
in Step 211, it is judged whether the measured level is under the level of two sheets or not;
in Step 212, if it is judged that the measured level is not under the level of two sheets, it is judged whether or not all of the sheets are fed. If they are not, Step 208 is repeated; and
in Step 213, if it is judged that all of the sheets are fed in the former step, the apparatus stops its operation.
However, in such a conventional apparatus, several problems arise. For example, it is required that the signal level from the light sensor be preset by adjusting its volume with a standard sheet of paper set in the sensor. Ordinarily, the preset signal level is fixed.
Furthermore, since paper dust becomes attached to the light sensor or its performance is degraded by the elapse of time, its light signal is lowered. As a result, the apparatus makes incorrect decisions. In addition, if the base color of paper is other than white, its light transmittance is different from that of white paper of the same thickness. Therefore, it is impossible to set a switch based only on the thickness of paper.